Gathering Data (C11-13 BVD) C12: Sampling and Surveys

* When we want information about a population, it is rarely possible to get data from every member. * What might you use as a sampling frame to sample the U.S. adult population?

* The design of a statistical study shows bias if it systematically favors certain outcomes. * If asked on an AP exam how a design shows bias, explain the direction of bias, such as the following: * “This sample question will likely overestimate the number of students who eat lunch at school because it was asked in the cafeteria at lunchtime.” * Bad sampling: convenience – easiest to reach Voluntary response – people volunteer to participate rather than being selected from a sampling frame

* 1. SRS * 2. Stratified – SRS within each of several groups of similar individuals. (9 th, 10 th, 11 th,12 th ) or (male, female), etc. * 3. Cluster – divide population into groups that mirror the population. Use SRS procedures to pick clusters, then sample ALL individuals in each chosen cluster. (Coronado, Harrison, Lewis Palmer student councils chosen as clusters to represent all student councils in county) * 4. Multistage – some combination of above methods

* SRS: simple to execute, all individuals and sample are equally likely. But, random variation means certain subgroups may be over or underrepresented (disproportionate numbers of males, or seniors, etc.) * Stratified – all individuals are equally likely to be chosen, and strata will all be represented. May be a little harder to execute than an SRS. * Cluster – may be more convenient to execute than SRS or Stratified, but if clusters are not all truly representative of the population, some subgroups may be over/underrepresented.

* In a recent census, Wyoming had 25,500 9 th graders, 24, th graders, 22, th graders and 18, th graders for a total of 91,000 students in high school. There are 91 public high schools in the state. * Explain how to use table/technology to find an SRS of 400 students, a sample stratified by grade, and a cluster sample.

* One line of random number table shown below: *

* One line of random number table shown below: *

* One line of random number table shown below: *

* Undercoverage – sampling frame does not equal population – some groups/individuals in population left out * Nonresponse – individual chosen declines or cannot be contacted * Response – systematic pattern of incorrect responses wording of questions, questionnairre design